Although insect herbivores and their host plants dominate much of terrestrial biodiversity, the factors that determine host plant specificity and host range of insect herbivores are incompletely understood. Biochemical adaptations of the larval stages to neutralize host plant chemical defenses are crucial for host plant colonization. The cabbage moth, Plutella xylostella (Lepidoptera: Plutellidae), utilizes a broad range of host plants from the Brassicaceae and related families, which are all defended by the highly diversified glucosinolate-myrosinase system. In this insect, we have previously identified larval glucosinolate sulfatase (GSS) as essential for host plant adaptation. We have also shown that the three GSS genes of P. xylostella encode enzymes with different substrate spectra and respond differentially to host plant glucosinolates. Because of the profound knowledge about plant glucosinolate-based chemical defense and the ability to determine functional properties of insect GSS genes and gene products, the interaction between Plutellids and their host plants provides an excellent opportunity to decipher the functional, ecological and evolutionary bases of insect host plant adaptation. In this project, we address the dynamics of change in GSS function over evolutionary times by exploring the biochemical properties of the encoded enzymes in a broad range of Plutellids with different degrees of specialization in relation to host plants and their glucosinolate-based chemical defense. We expect that insect species with a broad and diversified host plant spectrum should possess GSS activity towards a broad range of glucosinolates. By contrast, species with a narrow host spectrum (and particularly monophagous species) should possess GSS activity with a narrow substrate spectrum. By reconstructing ancestral GSS genes and characterizing extant and ancestral GSS enzymes, we will determine historical change in GSS function and direction of host range changes. The results of this project will shed light on several long-standing questions about the interaction between plants and herbivorous insects: Why do herbivorous insects feed on some plant species and not on others? Why do some insects have a broad host range while close relatives have a narrow host range? Which factors determine host plant specificity and host plant range?